1. Field of the Invention
The invention relates to RFID systems, in particular to RFID systems for security or safety applications. More specifically, the present invention relates to a method, system and an apparatus for reducing the reaction time of an RFID system by repeatedly disabling and enabling the carrier field of an RFID reader.
2. Description of the Related Art
RFID is an acronym for Radio Frequency Identification. RFID is one member in the family of Automatic Identification and Data Capture (AIDC) technologies and is a fast and reliable means of identifying just about any material object. It relies on storing and remotely retrieving data. Primarily, the two main components involved in a radio frequency identification system are the transponder (tags that are attached to the object) and the interrogator (RFID reader). Communication between the RFID reader and tags occurs wirelessly and generally does not require a line of sight between the devices. Some transponders can be read from several meters away.
Most RFID transponders contain at least two parts. One is an integrated circuit (IC) for storing and processing information, modulating and demodulating a (RF) signal, and other specialized functions. The second is an antenna for receiving and transmitting a signal. Accordingly, an RFID transponder, which is considered as a next generation barcode, is a miniscule microchip that is attached to an antenna. The transponders come in a wide variety of sizes, shapes, and forms and can be read through most materials with the exception of conductive materials like water and metal. With modifications and positioning, even these obstacles can be overcome.
Tags may be active or passive. Passive tags are generally smaller, lighter, and less expensive than those that are active, they can be applied to objects in harsh environments, they are maintenance free, and they will last for years. These transponders are only activated when within the response range of a reader. The RFID reader emits a low power electromagnetic wave field which is used to power up the tag to facilitate passing on of any information that is contained on the transponder chip.
Active tags differ in that they incorporate their own power source, wherein the tag is a transmitter rather than a reflector of radio frequency signals, which enables a broader range of functionality, like programmable and read/write capabilities.
An RFID reader typically contains a transmitter and receiver module, a control unit, and a coupling element (antenna). Such a typical reader has three main functions: energizing, demodulating, and decoding. In addition, readers can be fitted with an additional interface that converts the radio waves returned from the RFID tag into a form that can then be passed on to another system like a computer, microcontroller unit, or any programmable logic controller.
In safety engineering, the RFID technology is used, in particular, for non-contact switch devices, e.g., for door monitoring. Here, a predetermined device pair composed of a passive transponder and a reader may be attached to the door and a fixed point of reference, respectively. This facilitates monitoring whether a door is closed or open, e.g., in an environment where it is crucial that all doors of the room are closed before safety critical operations are performed inside the room. RFID may also be utilized for access control and general security purposes, providing authentication tags for opening a security door or enabling operation of a specific machine.
According to a conventional method of employing RFID technology, the RFID reader generates a carrier field. Once an active or passive transponder with an authorized identification code enters the response range of the RFID reader, it either transmits an electromagnetic signal comprising the ID code or influences the carrier field, for example, by means of load modulation. The reader attempts to determine a code from the response signal received from the transponder. Once a code is identified, the reader determines whether the identified code is valid. This may conventionally be accomplished by comparing the identified code with a code stored in the reader device. The process is repeated as long as the carrier field is powered. If a valid code is determined, the controller module of the RFID reader or an MCU connected thereto provides a predefined control signal. This control signal may be utilized to, for example, unlock a door or indicate to a monitoring system whether a specific door is open or closed.
In case of a passive transponder, the transponder is only provided with sufficient energy from the RF field of the reader if the transponder is within a certain energy transfer range of the reader. Once enough energy is transferred from the RF field to the transponder, the transponder starts to provide a response signal comprising its identification code to the RFID reader. If the transponder is within a response range of the reader, the provided response signal allows to determine the identification code as valid. Accordingly, once the transponder has received enough energy to start to transmit its code and has entered the response range of the reader, the reader device will be able to determine the code as valid and facilitate the provision of the assigned control signal.
This kind of RFID system suffers from the basic problem that in the border area of the response range of the reader device, transponders change their state from a state of not being able to transmit a valid code to a state of providing a response signal allowing to detect the valid identification code of the transponder, and vice versa. Conventionally, the identification code of the transponder is programmed into the transponder integrated circuit. Normally, the code consists of one bank with four bytes. These bytes are emitted for example in Manchester code once the transponder is in the carrier field of the RFID reader and has sufficient energy. The Manchester code consists of a header byte, and the code which generally consists of at least four bytes is sent continuously by repeating the sequence of header and code bytes.
If the distance between the transponder and the RFID reader is too large, i.e. the transponder is not located within the response range of the reader, but the transponder is provided with sufficient power to send the data, the control unit or MCU associated with the RFID reader cannot read the valid data. If the transponder is approaching the reader, the received code is read and may be stored until a first header byte is validly detected. Subsequently, the control unit or MCU can read the code and validate the code. This lengthy procedure negatively impacts the reaction time of the RFID system.